Displacement type pilot valve

ABSTRACT

A displacement type pilot valve in which an exhaust valve and an air supply valve are both enabled free to get into and out of an output pressure chamber, thereby improving the response performance of the valve rod provided on its opposite ends with said exhaust valve and air supply valve.

. United States Patent 11 1 1111 3,762,444 Nagata et al. 1 Oct. 2, 1973[54] DISPLACEMENT TYPE PILOT VALVE 2,767,739 10/1956 Hughes et al.138/46 3,054,426 9 1962 F 't t' l. 1. 138 46 [751 Inventors: Take N388?lch'ro 3 135 994 6/1964 sli in nirf 138/46 of Hitachi; Mimi" Ai,Katsuta, of 3,030,778 4/1962 Taylor 137/625.66 x Japan 3,470,909 10/1969Reis 1 137/6256 [73] Assignee: Hitachi, Ltd., Tokyo, Japan [22] Filed:Mar. 30, 1971 Primary ExaminerHenry T. Klinksiek [2H Appli No: 129,531Attorney-Craig, Antonelli, Stewart 8L Hill [30] Foreign ApplicationPriority Data Apr. 3, 1970 Japan 45/27839 ABSTRACT [52] U.S. Cl.l37/625.66, 138/46 A displacement type pilot valve in which an exhaust[51] Int. Cl Fl6k 11/07 valve and an air supply valve are both enabledfree to [58] Field of Search 137/6256, 625.66; get into and out of anoutput pressure chamber, 138/43, 45, 46; 261/69 R thereby improving theresponse performance of the valve rod provided on its opposite ends withsaid ex- [56] References Cited haust valve and air supply valve.

UNITED STATES PATENTS 2,394,663 2/1946 Carlson et al. 138/45 UX 9Claims, 22 Drawing Figures Pmimmw 2 I915 SHEET l [1F 5 FIG:

91; Z COWERT/NG FLA/PER P/V PILOT ELEMENT NOZZLE VALVE P0 FEEBACKELEMENT v 5 #4 LOOP MW 6! INVENTORS 'PmEo NHCYHTH, IZCJ-HRO KIMURH HNDNufsuo cu ATTORNEYS DISPLACEMENT TYPE PILOT VALVE BACKGROUND OF THEINVENTION 1. Field of the Invention The present invention relates to adisplacement type pilot valve used as a pneumatic amplifier in apneumatic measuring and controlling instrument.

2. Description of the Prior Art Two kinds of pilot valves have beenknown in the art one of which is a displacement type pilot valve and theother of which is a force-balance-type pilot valve. Now the differencebetween the displacement type and the force balance type will bedescribed hereinbelow.

FIG. I shows a principle and basic construction of the prior artdisplacement type pilot valve, and FIG. 2 shows a block diagramrepresenting the performance of the pilot valve shown in FIG. 1.

Referring to FIG. 1, the reference numeral 1 shows a casing of the pilotvalve, 2 shows a pressure receiving member such as a diaphragm formoving by virtue of input pressure P a valve rod 9 and an exhaust valveportion 3 integrally fixed to the valve rod 9 to form an air passage onthe exhaust side, 4 shows an air supply valve interposed between thevalve rod 9 and the push spring 11 to form an air passage on the airsupply side, 5 shows an exhaust valve seat to form an exhaust airpassage together with the exhaust valve 3, 6 shows a supply valve seatto form an air supply passage together with the supply valve 4, 7 showsan input pressure chamber, 8 shows an air supply chamber, 12 shows anoutput chamber for transmitting an output pressure P,,, and thereference numeral 10 shows anexhaust chamber leading to the atmosphericpressure P In the construction as described above, an input pressure Iis exerted on the pressure receiving member 2 and is converted into aforce F,, and the force F, inturn acts against the spring force(composed force of the spring force of the push spring 11 and theelastic force of the diaphragm 2) and move the exhaust valve 3 and thesupply valve 4. By the movement of the supply valve and exhaust valve,the spacing between the valves 3 and 4 and the valve seats 5 and 6varies which results in variation in the amount of flow from the supplychamber 8 to the output pressure chamber 12 and the amount of flow fromthe output pressure chamber 12 to the exhaust chamber 10. Based on thevariation in the amount of flow, the output pressure'P varies. The pilotvalve having such construction and function as described above is calleddisplacement type pilot valve.

It will be noted that in the displacement typepilot valve the outputpressure P is determined by the difference between the amount ofthe airsupply from the spacing formed with the supply valve 4 and the supplyvalve seat 6 and the amount of the exhaust flow passing through thespacing formed with the exhaust valve 3 and the exhaust valve seat 5.

Referring to FIG. 2 showing a block diagram of the performance of thepilot valve shown in FIG. 1, the reference character P shows an inputpressure exerted on the pressure receiving member 2, A shows aneffective area of the pressure receiving member 2, S shows a springconstant (kg/cm), KO shows a coefficient represented by the ratio of theoutput pressure and the displacement of the valve for the outputpressure. As apparent from the drawing, one of the features of thedisplacement type pilot valve is that there is no feedback means.

FIG. 3 shows a principle and basic construction of the prior artforce-balance-type pilot valve, and FIG. 4 shows a block diagramrepresenting the performance of the pilot valve shown in FIG. 3.

Now, when an input pressure P is exerted on the input pressure receivingmember 202 in the force balance type pilot valve as shown in FIG. 3, theinput pressure P is converted into the force F,, and the force F, actsagainst the composed spring force (composed force of the elasticity ofthe input pressure receiving member 202, the elasticity of the outputpressure receiving member 204 and the spring force of the push spring206) and moves the exhaust valve rod 203 and the supply valve 205.

When the input pressure P becomes positive with respect to the balancedpressure by the movement of the exhaust valve 203 and the supply valve205, a spacing is made between the supply valve 203 and the supply valveseat 205 and no spacing is made between the valve 205 and the exhaustvalve seat 208. On the contrary, when the input pressure becomesnegative with respect to the balanced pressure, a spacing is madebetween the supply valve 205 and the exhaust valve seat 208 and nospacing is made between the supply valve 205 and the supply valve seat207. Now, in the case that the input pressure P becomes positive withrespect to the balanced pressure, the supply valve 205 is pusheddownward in the drawing and the supply pressure P makes the air flowfrom the supply pressure chamber 211 to the output pressure chamber 210and the output pressure P increases. The output pressure P is exerted onthe output pressure receiving member 204 and the pressure receivingmember 204 is urged in the opposite direction to that of the action ofthe supply pressureP whichresults in closing of the supply valve 205 andthe supply valve seat 207. Then, when the force F, made by the supplypressure P exerted on the supply pressure receiving member 202 becomesequal to the force F made from the output pressure P exerted on theoutput pressure receiving member 204,

the supply valve 205 is brought into closed contact with thesupply valveseat 205, the supply valve seat 207, and the exhaust valve seat 208.Thus, the pilot valve comes into the balanced condition. y

Further, when the input pressure becomes negative with respect to thebalanced pressure, a spacing is made betweenthe supply valve 205 and theexhaust valve seat 208 as described above, and the air within the outputpressure chamber 210 is discharged into the atmosphere through theexhaust pressure chamber 209. Then, when the force F, made by the supplypressure P exerted on the supply pressure receiving member 202 becomesequal to the force F made by the output pressure P exerted on the outputpressure receiving member 204, the supply valve 205 is brought intocontact with the supply valve seat 207 and the exhaust valve seat 208.Thus, the pilot valve is made to be balanced. In short, the outputpressure P is determined by th difference between the amount of the airsupply from the spacing formed with the supply valve 205 and the supplyvalve seat 207 and the amount of the exhaust flow passing through thespacing formed with the exhaust valve seat 208 and the supply valve 205in the force balance type pilot valve. In this type of the pilot valve,the force exerted on the input pressure receiving member is balancedwith the spring force of the input pressure receiving member, the supplyvalve push spring and the output pressure receiving member. Anotherfeature of the force balance type pilot valve is that when the pilotvalve is balanced, the supply valve 205 is brought into closed contactwith the supply valve seat 207 and the exhaust valve seat 208. Thebalanced condition referred to herein means the condition where F,=Fwherein F, (input pressure P X (effective area of the input receivingmember 202 A and F (output pressure P X (effective area of the outputreceiving member 204 A FIG. 4 is a block diagram representing theperformance of the valve shown in FIG. 3. Referring to FIG. 4, thereference character S shows a spring coefficient of the whole system ofthe pilot valve shown in FIG. 3 and the reference character KQ shows acoefficient of the displacement of the valve for the output pressure Pwhich is determined by the ratio of the output pressure and thedisplacement of the valve for the output pressure. The referencecharacter A shows an effective area of the output pressure receivingmember. One of the features of the force balance type pilot valve isthat there is a feedback effect.

As described above, the displacement type pilot valve has no feedbackmeans and accordingly is of the bleed type. On the other hand, the forcebalance type pilot valve has a feedback means and accordingly is of thenonbleed type.

Based on the above difference in the feedback system, the amount ofconsumption of the air is different in the type of pilot valve. That is,in the displacement type pilot valve the air consumption is about 0.2 to1.5 Nm /h, while it is about 0.02 to 0.4 Nm /h in the force balance typepilot valve. With respect to the amount of the air consumption, theforce balance type pilot valve is more advantageous, accordingly.

However, in the sense of construction, the force balance type is lessadvantageous than the displacement type one, since a complexconstruction is required in the force balance type pilot valve such asan output receiving member 204 provided between the output pressurechamber 210 and the exhaust pressure chamber 209 and a bottomed slotprovided in the exhaust valve rod connected to the exhaust pressurechamber 209.

SUMMARY OF THE INVENTION A principal object of the present invention isto provide a displacement type pilot valve having a simple constructionas of the prior art pilot valve of this type and at the same time havingan effect of reducing the amount of air consumption.

In addition to the aforementioned disadvantage that the amount of airconsumption is large in the displacement type pilot valve, this type ofpilot valve is also disadvantageous in that the response speed is slowand further the response time in case that the output pressure increasesis different from that in case that the output pressure decreases.

Therefore, the second object of the present invention is to provide adisplacement type pilot valve which has a quick response.

The third object of the present invention is to provide a displacementtype pilot valve which has a constant response time of the valve in bothcases where the output pressure increases and decreases.

The fourth object of the present invention is to provide a displacementbalance type pilot valve which will not be affected by the fluctuationin the supply pressure.

In accordance with the present invention, there is provided adisplacement balance type pilot valve comprising a casing including aninput pressure chamber to which the input is led in, an exhaust pressurechamber leading to the atmosphere and an output pressure chambercomparted from the input pressure chamber with a diaphragm, an airsupply pressure chamber to which air is supplied and an output pressurechamber connected to the air supply pressure chamber and the exhaustpressure chamber for transmitting the output pressure out, a valve rodintegrally connected with a valve to be disposed in the passage from theoutput pressure chamber to the exhaust pressure chamber at an endthereof and integrally connected with a valve to be disposed in thepassage from the air supply pressure chamber to the output pressurechamber at the other end thereof, said valve to be disposed in thepassage from the output pressure chamber to the exhaust pressure chamberbeing secured to said diaphragm, and a push spring forwarding said valverod to said diaphragm, wherein the output pressure chamber and the valverod are so constructed as to satisfy the formulae d d,, and l l where dis a diameter of the output pressure chamber, I is the height thereof, dis a diameter of the valve rod, and I is the length of the valve rodincluding the valves fixed at the both ends thereof.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a vertical cross sectionalview of the conventional displacement type bleed pilot valve,

FIG. 2 is a block diagram showing the performance of the pilot valveshown in FIG. 1,

FIG. 3 is a vertical cross sectional view of the conventional forcebalance type nonbleed pilot valve shown in schematic representation,

FIG. 4 is a block diagram showing the performance of the pilot valveshown in FIG. 3,

FIG. 5 is a vertical cross sectional view showing an embodiment of thepilot valve in accordance with the present invention,

FIG. 6 is an enlarged detail view in section of the valve and valve seatportion of the pilot valve shown in FIG. 5,

FIGS. 7A through 7C show the operation of the valve portion of the pilotvalve shown in FIG. 5,

FIGS. 8A through 8D show different modified embodiments of the pilotvalve in accordance with the present invention,

FIG. 9 is a schematic vertical sectional view of still anotherembodiment of the pilot valve in accordance with the present invention,

FIG. 10 is a graphical representation showing characteristics of thedisplacement type semibleed pilot valve in accordance with the presentinvention in comparison with the conventional displacement type bleedpilot valve,

FIG. 11 is a diagrammatic representation showing a feedback systemincluding a pilot valve employed in a pneumatic measuring instrument ora controller,

FIG. 12 is a schematic sectional view of the pilot valve in accordancewith the present invention shown together with the relevant system inone application of the pilot valve of the present invention,

FIGS. 13A through 13C are graphical representation of thecharacteristics of the pilot valve of the invention showing theinfluence of the flapper and nozzle to the output pressure of the pilotvalve according to the fluctuation of the air supply,

FIG. 14 is a graph showing a characteristic curve representing therelation between the air supply pressure and the output pressure in thepilot valve, and

FIG. 15 is a graphical representation of the characteristic curves ofthe pilot valve in accordance with the present invention not affected bythe fluctuation in the air supply pressure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 5 shows an embodiment ofthe pilot valve in accordance with the present invention in a sectionalview. Referring to FIG. 5, the reference numeral 501 shows a casing ofthe valve, 502 shows a pressure receiving member such as a diaphragm formoving a valve rod 503 by the input pressure P 504 shows an exhaustvalve to form an air passage on the exhaust side, 505 shows an airsupply valve forming an air passage on the air supply side, 506 anexhaust valve seat forming an exhaust passage together with the exhaustvalve 504, 507 an air supply valve seat to form an air supply passagetogether with the supply valve 505, 508 an input pressure chamber, 509an air supply pressure chamber, 510 an output pressure chamber, 511 anexhaust pressure chamber, 512 a push spring to push the valve rod 503toward the diaphragm, the reference character P shows a supply pressure,P an input pressure, P an output pressure and P A shows atmosphere.

FIG. 6 shows a detailed construction of the valve and valve seat portionof the pilot valve shown in FIG. 5 and FIGS. 7A to 7C show the operationof the valve portion. FIG. 7A shows the valve portion in the state wherethe input pressure is not yet balanced, FIG. 7B shows the valve portionin the state of balancing, andFlG. 7C shows the valve portion in thestate that the excessive input pressure is exerted on the valve portion.

In FIGS. 6 and 7A to 7C, the reference numeral 513 shows an air supplypassage formed with an air supply valve 505 and the air supply valveseat 507,514 shows an exhaust leakage spacing formed with the valve rod503 and the valve seat 519 to determine the amount of bleed at the timeof balancing, 515 (FIG. 7) shows an air supply leakage spacing formedwith the valve rod 503 and the valve seat 519 to determine the supplyleakage before the balancing, and the reference numeral 516 shows an airsupply passage formed with the air supply valve 505 and the supply valveseat 507 for passing the supplied air at the time when an excessiveinput pressure is exerted on the valve portion.

Further, one of the features of the valve of the present invention isthat the dimension of the valve portion is made so that the relationshipas d d, and 1 g 1 is satisfied, where d, is the diameter of the outputpressure chamber 510 of the casing 501, I is the height thereof, d isthe diameter of the valve rod, I is the length of the valve rodincluding the valves fixed to the end portions thereof as shown in FIG.6.

Now, when the input pressure P enters into the input pressure chamber508 and the valve is brought into the balanced condition, the diaphragm502 pushes the valve rod 503 downward by the pressure. Then the valverod 503 is stopped at the position where the clasticity of the diaphragm502 is balanced with the spring force of the spring 512. Such a balancedsituation is shown in FIG. 7B. Thus, the amount of the air supply Q andthe exhaust leakage Q, are determined by the size of the air supplypassage 513 and the exhaust leakage spacing 514 which are in turndetermined by the position of the valve rod 503. Thus, the outputpressure I is controlled by the input pressure P While the inputpressure P is not yet balanced with the air supply, the air supplypressure P is exerted on the sectional area of the valve rod 503 and thepushes the valve rod 503 upward cooperating with the spring 512, and theair supply passage 513 forms an air supply leakage spacing 515 and theexhaust passage 518 as shown in FIG. 7A. Under such a situation, theamount of the air supply is made extremely small and the amount of theexhaust air is increased, which results in improvement in the responseof the valve at the time of the exhaust.

In the event that an excessive input is provided into the input pressurechamber 508, the air supply passage 513 is broadened as indicated by 516in FIG. 7C and the exhaust leakage spacing 514 increases its resistanceand accordingly the exhaust leakage is decreased by a similar effect asthat described above in the explanation of the umbalanced situation.

By the above operation, that is by the construction wherein the exhaustpassage is normally narrowed by the valve, the amount of the airconsumption is made extremely small. Comparing the pilot valve inaccordance with the present invention with the conventional displacementtype bleed pilot valve, with the same response being maintained, theamount of the air consumption of the pilot valve of the presentinvention is able to be made less than one tenth of the conventionalone. Thus, the primary object of the present invention can be fulfilled.Since the exhaust valve and the air supply valve are made to be freelymoved in the output pressure chamber, the valve rod is movable inproportion to the height of the input pressure P Further, the stroke ofthe valve movement can be made very large. Based on the aboveconstruction of the valve rod, the response time is shortened withrespect to the conventional pilot valve. Thus, the second object of thepresent invention can be accomplished.

Further, if the valve rod is made of light metals, it is advantageous inthe effect that the valve stroke can easily be made large. Since theamount of the exhaust air is constant whether the input pressure Pincreases or decreases, there is no difference in the operating time orresponse time of the valve rod between the response time when the valverod rises upward and that when it moves downward. Thus, the third objectof the present invention can be carried out.

FIG. 10 shows a graph of the characteristic curves of the conventionaldsplacement type bleed pilot valve and a pilot valve of the presentinvention of the semibleed displacement type (The amount of bleed isremarkably small in comparison with the conventional bleed type valve.).The reference character Q indicates the amount of the air flow on theoutput side, 0,, indicates the amount of exhaust (i.e. bleed amount), Aindicates a characteristic curve of the relationship between P and O inthe conventional pilot valve, B indicates a characteristic curve of therelationship between P and O in the pilot valve of the presentinvention, (a) indicates a P Q curve of the conventional pilot valve,and (b) indicates a P -Q curve of the pilot valve of the presentinvention. As apparent from A in FIG. 10, there is no increase in Q evenwhere the P,,, is larger than a definite value, and 0,, is decreased tozero where the P is increased.

The reason for the above described behavior of the valve of the presentinvention is that in response to the stabilizing of the input prssure Pthe exhaust valve por tion 3 shown in FIG. 1 closes the exhaust valveseat 5 and the amount of the exhaust air becomes zero in theconventional valve. On the other hand, in the pilot valve in accordancewith the present invention, the exhaust valve portion 504 and the airsupply valve portion 505 are made to freely enter and get out of theoutput pressure chamber 510 according to the variation in the inputpressure P Therefore, there is no shut-off action of the exhaust air inany situation as made in the prior art valve as described above. It willbe readily understood, therefore, that the response speed is higher inthis invention than the conventional one in the case that the inputpressure is exerted on the pilot valve under the equivalent condition.Further, since the amount of the air bleed is constant in the pilotvalve of the present invention as described above, the exhaust air willbe easily understood to be represented by the curve b.

FIGS. 8A through 8D show various embodiments of the valve portionemployed in the pilot valve in accordance with the present invention. Inthe various embodiments, the reference numeral 803 shows a valve rod,804 shows an exhaust valve portion, 805 shows an air supply valveportion, and the reference numeral 801 shows the casing of the pilotvalve. The shape of the valve and the output pressure chamber may be ofany type shown in the FIGS. 8A to 8D. It will be understood that variousother shape of the valve and the output pressure chamber may be employedin the variation and modification of the pilot valve of the presentinvention within the scope of the invention as defined in the claims.

FIG. 9 shows another embodiment of the pilot valve construction inaccordance with the present invention, in which the valve seat is madeof elastic material.

In FIG. 9, the elements or portions equivalent to those in the valveshown in FIG. 5 are all represented by the equivalent reference numeralsused in FIG. 5. A chamber enclosed by a couple of elastic members 905and 906 is an output pressure chamber. Beside this chamber, all theother components are equivalent to those in the valve shown in FIG. 5.It will be understood that any valve employing an exhaust valve portionor air supply portion made of elastic material is included within thescope of the invention.

Now the construction of the displacement balance type pilot valveaccomplishing the fourth object of the present invention will bedescribed.

Generally, in the pneumatic measuring instruments and controllers,fluctuation in the air supply pressure (ordinarily 1.2 to 1.6 kg/cm)results in fluctuation in the output pressure at the flapper, nozzle-andthe pilot valve. The measuring instruments or controllers are generallyprovided with a feedback loop as shown in FIG. 11 in order to improvethe linearity and the like. In FIG. 11, the reference character Zindicates an input signal, the reference numeral 111 shows a convertingelement, 112 a flapper nozzle, 113 a pilot valve] and 114 shows afeedback element. The reference character P shows the air supplypressure, P a nozzle back pressure, l the output pressure, and G, showsthe gain of this loop.

Normally, when the air supply pressure P is decreased, both the nozzleback pressure P and the output pressure P are also decreased. If thegain of the feedback loop G, is made sufficiently large, the variationin the output becomes smaller as represented by l/(l-l-G,). However, ifthe gain G, is made large, the system becomes unstable and thepossibility of self vibration happens. Accordingly, the value of thegain G, is limited. This is the same for the case where the air supplypressure is increased.

In the present invention, a pilot valve which will not be affected bythe fluctuation in the supply pressure can be provided with a simpleconstruction in a system employing a flapper nozzle and a pilot valve.That is, by properly designing the sectional area of the air supplyvalve of the displacement type pilot valve, the output pressure is madeto increase when the air supply pressure is decreased, and by theincrement the decrement due to the flapper nozzle is compensated. Evenwhen the air supply pressure is increased, the similar effect can beobtained.

In FIG. 12, the reference numeral 501 shows a casing, 531 an air supplypressure transmitting tube, 520 an air supply pressure guiding port, 523an output pres sure outlet, 524 an exhaust port, 525 a nozzle backpressure (input pressure) guiding port, 526 and 527 nozzle back pressuretransmitting tubes, 509 an air supply chamber, 512 a spring, 505 an airsupply valve, 503 a valve rod, 507 an air supply valve seat, 506 anexhaust air valve seat 504 an exhaust valve integrally fixed to thevalve rod 503, 510 an output pressure chamber, 508 an input pressurechamber leading to the nozzle back pressure, 502 a diaphragm, 528 achoke, 529 a nozzle, 530 a flapper, P an air supply pressure, P anoutput pressure, P,, atmospheric pressure, and P indicates a nozzle backpressure (input pressure). When the nozzle back pressure P enters intothe nozzle back pressure (input pressure) chamber 508, the diaphragm 502pushes, by the pressure, the exhaust valve 504, the valve rod 503 andthe air supply valve 505 upward in the drawing FIG. 12 and is stopped ata position where the pushing force is balanced with the spring force ofthe spring 512. According to the spacing between the air supply valve505 and the valve seat 507 and the spacing between the exhaust valve 504and the valve seat 506, at the stopped position, the amount of the airsupply, output flow and the exhaust flow are determined. Thus, theoutput pressure P is controlled by the nozzle back pressure P Unless theloaded side of the output pressure P is opened to atmosphere, thefollowing formula is satisfied in the balanced condition (I) where g(X)is a function representing the valve characteristic, X is a displacementof the valve, and P is an air supply pressure. And X is represented bythe following formula where S is a spring constant, A is an effectivearea of the diaphragm 502 and A is area determined by the diameter ofthe valve seat.

Further, in the case that the output pressure P varies by AP when theair supply pressure P varies by AP the relationship between AP and AP isrepresented as follows (3) where AP is a fluctuation of the outputaccompanying the increase of the air supply due to the fluctuation inair supply pressure AP and AP is a fluctuation of the nozzle backpressure accompanying the fluctuation in the air supply pressure AP andK is a gain of the pilot valve.

Though the second term of the formula (2) and the second and third termsof the numerator of the formula (3) have been neglected heretofore, thesecond term of the formula (2) can be made equal to the first term byproperly designing the area A For example, in the case that the airsupply pressure P is varied by AP the nozzle back pressure P varies byAP as shown in FIG. 13A. By the variation AP of the nozzle backpressure, the output pressure P is varied by AP as shown in FIG. 138.Accordingly, as shown in FIG. 13C, by the variation AP of the air supplypressure P the output pressure P of the pilot valve is varied by AP dueto the influence of the nozzle back pressure. On the other hand, the airpressure fluctuation AP brings about fluctuation of P as shown in FIG.14.

Size of the valve seats 507 and 506 and the diameter of the choke 528and the orifice of the nozzle 529 are also the factors of thefluctuation in the output fluctua tion due to the air supply pressurefluctuation. As shown in FIG. 15, fluctuation in the air supply pressurecauses fluctuation in the nozzle back pressure AP In the case that therelationship of the fluctuation AP in the output pressure caused by theabove fluctuation in the input pressure and the air supply pressurefluctuation AP is represented by the curve 101 in the graph in FIG. 15,the relationship between the air supply pressure P and the outputpressure I" becomes as the curve 100 shown in FIG. 15 if a pilot valveis made to have the characteristic as represented by the curve 102 inFIG. 15 by designing properly the effective area A determined by thediameter of the valve seat so that the first term of the formula (2) maybe cancelled thereby. Thus, a pilot valve in which the output pressure Pis maintained constant regardless of the fluctuation of the air supplypressure P within the practical range of the pressure can be obtained asshown a fat line L in FIG. 15.

As described above, in accordance with the present invention there isprovided a pilot valve of this kind in which the output pressure is notaffected by the fluctuation in the air supply pressure only by designingthe sectional area of the supply valve as described above.

Further, in the pilot valve in accordance with the present invention,the volume of the whole valve can be reduced to one fifth as small asthe conventional valve since the shape of the valve is made compact bybeing made a sluice shape. In addition, since the stroke of the movementof the valve can be made large, the efficiency of transmitting orfeeding out the output air can be increased.

As hereinabove described, in accordance with the present invention, theair consumption is remarkably reduced to improve the valve in the senseof economy, and further the response performance is improved and theresponse time is made constant whether the input pressure increases ordecreases. Thus, the improvement of the displacement type semi-bleedpilot valve brings about an improvement in the stability in the processcontrol and improvement in the controlling ability. Furthermore, in asystem employing a flapper nozzle and a pilot valve, an improved pilotvalve of this kind can be obtained in accordance with the presentinvention in which in the range where the value of AP /AP isapproximately constant the output pressure is not affected by thefluctuation in the air supply pressure by designing the air supply valve505 and the effective sectional area A of the spacing at the valve seat507 to satisfy the relationship as A =P A /P where P is a nozzle backpressure, A is an effective area of the diaphragm 502, and P is apredetermined air supply pressure.

What is claimed is:

l. A displacement type pilot valve comprising a casing including aninput pressure chamber into which input pressure is introduced, an airsupply pressure chamber into which an air supply is introduced, anoutput pressure chamber for receiving the air supply from the supplypressure chamber and transmitting an output pressure in proportion tothe input pressure, and an exhaust chamber separated from the inputpressure chamber with a diaphragm, the exhaust chamber being incommunication with the output pressure chamber and with atmosphericpressure; a valve rod means including an exhaust valve fixed at an endthereof to the diaphragm and forming an exhaust passage leading from theoutput pressure chamber to the exhaust pressure chamber, an air supplyvalve forming an air supply passage leading from the air supply pressurechamber to the output pressure chamber and a valve rod extending betweenthe exhaust valve and the supply valve; and a valve rod pushing springto urge the valve rod disposed in the air supply pressure chamber towardthe diaphragm, the output pressure chamber having at least one of a wallportion and a valve seat in the region of the exhaust passage forcooperation with the exhaust valve and at least one of a wall portionand a valve seat in the region of the air supply passage for cooperationwith the air supply valve, the supply valve having an outer diameterwhich is slightly smaller than the inner diameter of the at least onecooperating valve seat and wall portion and the exhaust valve having anouter diameter which is slightly smaller than the inner diameter of theat least one cooperating valve seat and wall portion to provide a flowof air between the output pressure chamber and at least one of the airsupply chamber and the exhaust chamber in all positions of the exhaustand supply valves.

2. A displacement type pilot valve as defined in claim 1 wherein atleast one of the exhaust valve and the air supply valve of said valverod is normally opened to a degree corresponding to the input pressure.

3. A displacement type pilot valve as defined in claim 1 wherein saidvalve rod is made of light metals.

4. A displacement type pilot valve as defined in claim 1 wherein theformula A P A IP is satisfied, where A is an effective sectional area ofthe air supply valve and the spacing between the supply valve and thevalve seat therefor, P is an input pressure, A is an effective area ofthe diaphragm and P is an air supply pressure.

5. A displacement type pilot valve comprising a casing including aninput pressure chamber into which input pressure is introduced, an airsupply pressure chamber into which an air supply is introduced, anoutput pressure chamber for receiving the air supply from the supplypressure chamber and for transmitting an output pressure in proportionto the input pressure, and an exhaust chamber separated from the inputpressure chamber by a diaphragm, the exhaust chamber communicating withthe output pressure chamber and with atmosphere, movable valve meansconnected to the diaphragm and extending from the exhaust chamberthrough the output chamber and into the air supply chamber, spring meansarranged in the air supply chamber for biasing the valve means extendinginto the air supply chamber toward the diaphragm, the valve meansventing a portion of the air supply to atmosphere in all positions ofthe valve means, and the valve means being freely movable with respectto the output pressure chamber.

6. A displacement type pilot valve as defined in claim 5, wherein themovable valve means includes an exhaust valve having one end thereofsecured to the diaphragm, an air supply valve being biased by the springmeans, and a valve rod extending between the exhaust valve and the airsupply valve,

7. A displacement type pilot valve as defined in claim 6, wherein atleast one of the exhaust valve and the air supply valve varies at leastone of the portion of the air supply vented to atmosphere and theportion of the air supply provided as an output of the output pressurechamber in accordance with the input pressure.

8. A displacernnnt type pilot valve as defined in claim 6, wherein theoutput pressure chamber is provided with at least one of a wall portionand a valve seat for cooperation with the exhaust valve, and at leastone of a wall portion and a valve seat for cooperation with the airsupply valve.

9. A displacement type pilot valve as defined in claim 8, wherein theexhaust valve and the air supply valve are dimensioned to define anarrow communicating passage between the output pressure chamber and atleast one of the exhaust chamber and the air supply chamber when theexhaust valve and the air supply valve are directly adjacent therespective cooperating wall portion and valve seat of the outputpressure chamber.

1. A displacement type pilot valve comprising a casing including aninput pressure chamber into which input pressure is introduced, an airsupply pressure chamber into which an air supply is introduced, anoutput pressure chamber for receiving the air supply from the supplypressure chamber and transmitting an output pressure in proportion tothe input pressure, and an exhaust chamber separated from the inputpressure chamber with a diaphragm, the exhaust chamber being incommunication with the output pressure chamber and with atmosphericpressure; a valve rod means including an exhaust valve fixed at an endthereof to the diaphragm and forming an exhaust passage leading from theoutput pressure chamber to the exhaust pressure chamber, an air supplyvalve forming an air supply passage leading from the air supply pressurechamber to the output pressure chamber and a valve rod extending betweenthe exhaust valve and the supply valve; and a valve rod pushing springto urge the valve rod disposed in the air supply pressure chamber towardthe diaphragm, the output pressure chamber having at least one of a wallportion and a valve seat in the region of the exhaust passage forcooperation with the exhaust valve and at least one of a wall portionand a valve seat in the region of the air supply passage for cooperationwith the air supply valve, the supply valve having an outer diameterwhich is slightly smaller than the inner diameter of the at least onecooperating valve seat and wall portion and the exhaust valve having anouter diameter which is slightly smaller than the inner diameter of theat least one cooperating valve seat and wall portion to provide a flowof air between the output pressure chamber and at least one of the airsupply chamber and the exhaust chamber in all positions of the exhaustaNd supply valves.
 2. A displacement type pilot valve as defined inclaim 1 wherein at least one of the exhaust valve and the air supplyvalve of said valve rod is normally opened to a degree corresponding tothe input pressure.
 3. A displacement type pilot valve as defined inclaim 1 wherein said valve rod is made of light metals.
 4. Adisplacement type pilot valve as defined in claim 1 wherein the formulaAV PNAD/PS is satisfied, where AV is an effective sectional area of theair supply valve and the spacing between the supply valve and the valveseat therefor, PN is an input pressure, AD is an effective area of thediaphragm and PS is an air supply pressure.
 5. A displacement type pilotvalve comprising a casing including an input pressure chamber into whichinput pressure is introduced, an air supply pressure chamber into whichan air supply is introduced, an output pressure chamber for receivingthe air supply from the supply pressure chamber and for transmitting anoutput pressure in proportion to the input pressure, and an exhaustchamber separated from the input pressure chamber by a diaphragm, theexhaust chamber communicating with the output pressure chamber and withatmosphere, movable valve means connected to the diaphragm and extendingfrom the exhaust chamber through the output chamber and into the airsupply chamber, spring means arranged in the air supply chamber forbiasing the valve means extending into the air supply chamber toward thediaphragm, the valve means venting a portion of the air supply toatmosphere in all positions of the valve means, and the valve meansbeing freely movable with respect to the output pressure chamber.
 6. Adisplacement type pilot valve as defined in claim 5, wherein the movablevalve means includes an exhaust valve having one end thereof secured tothe diaphragm, an air supply valve being biased by the spring means, anda valve rod extending between the exhaust valve and the air supplyvalve.
 7. A displacement type pilot valve as defined in claim 6, whereinat least one of the exhaust valve and the air supply valve varies atleast one of the portion of the air supply vented to atmosphere and theportion of the air supply provided as an output of the output pressurechamber in accordance with the input pressure.
 8. A displacemnnt typepilot valve as defined in claim 6, wherein the output pressure chamberis provided with at least one of a wall portion and a valve seat forcooperation with the exhaust valve, and at least one of a wall portionand a valve seat for cooperation with the air supply valve.
 9. Adisplacement type pilot valve as defined in claim 8, wherein the exhaustvalve and the air supply valve are dimensioned to define a narrowcommunicating passage between the output pressure chamber and at leastone of the exhaust chamber and the air supply chamber when the exhaustvalve and the air supply valve are directly adjacent the respectivecooperating wall portion and valve seat of the output pressure chamber.